Noise management for optical time delay interferometry

ABSTRACT

An integrated fiber interferometry interrogator for generating superimposed waves is disclosed. The system is optimized for efficiency and vibration attenuation. The system comprises an optical light source for generating a first signal, a first signal splitter which splits the first signal into a reference signal and an interrogation signal, optical modulators for modulating the signals, a fiber coupler connected to a fiber under test, an isolator, a circulator with a plurality of connections for directing the signals, a signal mixer for mixing the signals into superimposed waves, and photo diodes for receiving the superimposed waves.

CROSS REFERENCE TO RELATED PATENT APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.14/837,592 filed on Aug. 27, 2015, entitled “Noise Management ForOptical Time Delay Interferometry” which claims priority to U.S.Provisional Patent Application No. 62/042,989 filed on Aug. 28, 2014,entitled “System and Method for Electro Optical Modulation”, U.S.Provisional Patent Application No. 62/042,994 filed on Aug. 28, 2014,entitled “System and Method for Acousto-Optical Modulation”, U.S.Provisional Patent Application No. 62/042,997 filed on Aug. 28, 2014,entitled “System and Method for Fidelity up to 24,000 HZ”, U.S.Provisional Patent Application No. 62/042,999 filed on Aug. 28, 2014,entitled “Fiber-Optic Based Sensing System and Methods Using VirtualCorrelation Cells”, U.S. Provisional Patent Application No. 62/043,002filed on Aug. 28, 2014, entitled “System and Method for the ControlPanel”, U.S. Provisional Patent Application No. 62/043,004 filed on Aug.28, 2014, entitled “System and Method for the Hardware Control Panel andDiagnostics”, U.S. Provisional Patent Application No. 62/043,007 filedon Aug. 28, 2014, entitled “System and Method for Detection Logic”, U.S.Provisional Patent Application No. 62/043,009 filed on Aug. 28, 2014,entitled “System and Method for Telemetry Recording and Display”, U.S.Provisional Patent Application No. 62/043,015 filed on Aug. 28, 2014,entitled “System and Method for Audio Extension to Wave Convertor”, U.S.Provisional Patent Application No. 62/043,017 filed on Aug. 28, 2014,entitled “System and Method for Filtering High Low Band Pass”, U.S.Provisional Patent Application No. 62/043,023 filed on Aug. 28, 2014,entitled “System and Method for the Waterfall Display”, U.S. ProvisionalPatent Application No. 62/043,026 filed on Aug. 28, 2014, entitled“System and Method for Dynamic Characterization of Fiber Optic SensorArray”, U.S. Provisional Patent Application No. 62/043,029 filed on Aug.28, 2014, entitled “System and Method for Improved in Situ MeasurementsUsing Fiber Optic Sensor Array”, U.S. Provisional Patent Application No.62/043,031 filed on Aug. 28, 2014, entitled “System and Method forEnhanced Event Identification and Tracking Using Fiber Optic SensorArray”, U.S. Provisional Patent Application No. 62/043,034 filed on Aug.28, 2014, entitled “System and Method for Improved Identification,Classification, and Prediction of Micro-Seismic and Audible Events Usinga Fiber Optic Sensor Array”, U.S. Provisional Patent Application No.62/042,896 filed on Aug. 28, 2014, entitled “System and Method forDemodulating Rayleigh Backscattered Signals”, and U.S. ProvisionalPatent Application No. 62/199,098 filed on Jul. 30, 2015, entitled“System and Method for Fiber Optic Sensing”, which applications arehereby incorporated in their entirety by reference.

COPYRIGHT NOTICE

Contained herein is material that is subject to copyright protection.The copyright owner has no objection to the facsimile reproduction byanyone of the patent document or the patent disclosure, as it appears inthe United States Patent and Trademark Office patent file or records,but otherwise reserves all rights to the copyright whatsoever. Thefollowing notice applies to the software, screenshots and data asdescribed below and in the drawings hereto and All Rights Reserved.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to fiber optic sensing, and in particularto distributed acoustic sensing (DAS). More specifically, it relates toa system and methods that comprise an integrated fiber opticinterrogator and an embedded controller.

BACKGROUND

Fiber optic sensors are increasingly being used as devices for sensingquantities such as temperature, mechanical strain, displacements,vibrations, pressure, acceleration, rotations, or chemicalconcentrations. In fiber optic sensors, light is sent through an opticalfiber and the returning backscattered light is analyzed. Changes in theparameters of the returning light, as compared to the input light signalbaseline, may be measured and tracked.

By phase differencing the reflected signal with a reference signal,minute changes can be detected; these relate directly to the event thatis causing the laser signals to be reflected. As one example, acousticpressure waves in the vicinity of a fiber cable will impart microstrains on the fiber. These micro strains are proportional to theacoustic pressure waves, essentially imparting the frequency of theacoustic pressure wave into the back reflected signal; this is generallyreferred to a modulating a signal. Phase differencing the reflectedsignal allows the signal to be demodulated and the acoustic pressurewave reconstructed. This technology essentially turns a fiber opticcable into a microphone.

A growing usage application field for this technology is a fiber sensingsystem for remote downhole monitoring of oil wells. Other applicationfields include physical security, such as homeland security and bordermonitoring. The list of existing and potential applications for this newtechnology is long and continues to grow. Managing noise associated withthe fiber has proven difficult. For example, reduction of acousticsignals impinging on the system hardware that contribute to what istermed signal noise floor has been difficult.

SUMMARY OF THE INVENTION

Although the best understanding of the present invention will be hadfrom a through reading of the specification and claims presented below,this summary is provided in order to acquaint the reader with some ofthe new and useful features of the present invention. Of course, thissummary is not intended to be a complete litany of all of the featuresof the present invention, nor is it intended in any way to limit thebreadth of the claims, which are presented at the end of the detaileddescription of this application.

The following detailed description is directed to technologies for noisemanagement for optical time delay interferometry. In some examples,distributed fiber optic sensing is used to mitigate acoustic noise andnoise floor in an effort to increase the utility and/or flexibility ofthe sensing systems.

According to some examples, a distributed fiber optic sensing systemwith increased flexibility and/or utility is described.

In some configurations, a time-domain reflectometer is described whereinan optical fiber span is the object of the reflectometry, and providesoutput signals representative of acoustic pressure waves incident thespan.

Example configurations capable of providing acoustic wave signal sensinglengths of up to the total length a coherent signal that can be detectedand demodulated in a round trip are also described.

In some examples, acoustic wave signal sensing lengths of up to 40.0 kmmay be utilized. According to some configurations, a large plurality ofsensed events along the span may also be provisioned. In some examples,output signals in the form of a phase signal which varies linearly withthe acoustic pressure wave.

Other features of the present invention will be apparent from theaccompanying drawings and from the detailed description that follows.

Aspects and applications of the invention presented here are describedbelow in the drawings and detailed description of the invention. Unlessspecifically noted, it is intended that the words and phrases in thespecification and the claims be given their plain, ordinary, andaccustomed meaning to those of ordinary skill in the applicable arts.The inventors are fully aware that they can be their own lexicographersif desired. The inventors expressly elect, as their own lexicographers,to use only the plain and ordinary meaning of terms in the specificationand claims unless they clearly state otherwise and then further,expressly set forth the “special” definition of that term and explainhow it differs from the plain and ordinary meaning. Absent such clearstatements of intent to apply a “special” definition, it is theinventors' intent and desire that the simple, plain and ordinary meaningto the terms be applied to the interpretation of the specification andclaims.

The inventors are also aware of the normal precepts of English grammar.Thus, if a noun, term, or phrase is intended to be furthercharacterized, specified, or narrowed in some way, then such noun, term,or phrase will expressly include additional adjectives, descriptiveterms, or other modifiers in accordance with the normal precepts ofEnglish grammar. Absent the use of such adjectives, descriptive terms,or modifiers, it is the intent that such nouns, terms, or phrases begiven their plain, and ordinary English meaning to those skilled in theapplicable arts as set forth above.

Further, the inventors are fully informed of the standards andapplication of the special provisions of 35 U.S.C. § 112,116. Thus, theuse of the words “function,” “means” or “step” in the DetailedDescription or Description of the Drawings or claims is not intended tosomehow indicate a desire to invoke the special provisions of 35 U.S.C.§ 112, ¶ 6, to define the invention. To the contrary, if the provisionsof 35 U.S.C. § 112,116 are sought to be invoked to define theinventions, the claims will specifically and expressly state the exactphrases “means for” or “step for, and will also recite the word“function” (i.e., will state “means for performing the function of[insert function]”), without also reciting in such phrases anystructure, material or act in support of the function. Thus, even whenthe claims recite a “means for performing the function of . . . ” or“step for performing the function of . . . ”, if the claims also reciteany structure, material or acts in support of that means or step, orthat perform the recited function, then it is the clear intention of theinventors not to invoke the provisions of 35 U.S.C. § 112,116. Moreover,even if the provisions of 35 U.S.C. § 112, ¶ 6 are invoked to define theclaimed inventions, it is intended that the inventions not be limitedonly to the specific structure, material or acts that are described inthe examples, but in addition, include any and all structures, materialsor acts that perform the claimed function as described in alternativeexamples or forms of the invention, or that are well known present orlater-developed, equivalent structures, material or acts for performingthe claimed function.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present invention may be derived byreferring to the detailed description when considered in connection withthe following illustrative figures. In the figures, like-referencenumbers refer to like-elements or acts throughout the figures. Theexamples of the invention are illustrated in the accompanying drawings,in which:

FIG. 1 depicts the base configuration of an integrated fiber opticinterrogator and data logger.

FIG. 2 depicts the base configuration of FIG. 1 equipped with an exampleassortment of modular cards.

FIG. 3 depicts a first expansion card of FIG. 2—RF Mixing 700 and Analogto Digital Conversion (ADC), referred to herein as Card 1.

FIG. 4 depicts a second expansion card of FIG. 2—Signal Correlation 800,referred to herein as Card 2.

FIG. 5 depicts a third expansion card of FIG. 2—Control Logic 900 andData Logger, referred to herein as Card 3.

FIG. 6 is a graphical depiction of polarization.

FIG. 7 depicts a fourth expansion card of FIG. 2—Acoustic Intensity1000, referred to herein as Card 4.

FIG. 8 depicts a fifth expansion card of FIG. 2—Event Interrogation 1100and Demodulation, referred to herein as Card 5.

FIG. 9 depicts a sixth expansion card of FIG. 2—Noise Reduction 1200 andClassification, referred to herein as Card 6.

FIG. 10 depicts an example of a mechanical laser settings controlsystem.

FIG. 11 depicts a heater control circuit.

FIG. 12 depicts a laser interlock control circuit.

FIG. 13 depicts an example for the laser settings control system of FIG.10 wherein the mechanical controls are supplemented and/or replaced bymicroprocessor control.

FIG. 14 depicts an example on the laser settings control system of FIG.10 wherein both laser temperature and laser output power may bemonitored and the data may be fed back to a microprocessor.

FIG. 15A depicts a side view of an example fiber configuration forimproved acoustic sensitivity.

FIG. 15B depicts the example configuration of FIG. 15A with the additionof a catch basin.

FIG. 15C depicts a top view of FIG. 15C.

FIG. 15D depicts an example of FIG. 15A with the addition of a pressurerelief mechanism.

FIG. 15E depicts an example of FIG. 15A with the addition of more thanone pressure relief mechanism.

FIG. 16 depicts fiber sensitivity test segments buried with differentdepth relationships.

FIG. 17 depicts a parallel fiber configuration.

FIG. 18 depicts a triangle wave fiber configuration.

FIG. 19 is a frequency spectrogram for prototype Test A.

FIG. 20 is a frequency spectrogram for prototype Test B.

FIG. 21 is a frequency spectrogram for prototype Test C.

FIG. 22 is a frequency spectrogram for prototype Test D.

FIG. 23 is a frequency spectrogram for prototype Test E.

FIG. 24 is a frequency spectrogram for prototype Tests A-E.

FIG. 25 is a frequency spectrogram for prototype Tests 1-2.

FIG. 26 depicts a standard two-way beam splitter.

FIG. 27 depicts the beam splitter of FIG. 26 with a mandrel to reducenoise.

FIG. 28A depicts an example for an encased mandrel assembly.

FIG. 28B depicts the internal components of the mandrel assembly of FIG.28A.

FIG. 28C depicts a view of the cylinder around which the fiber iswrapped to form the mandrel of FIG. 28A.

FIG. 28D depicts an example for a securing mechanism for locking thecylinder within the mandrel assembly of FIG. 28A after the fiber hasbeen wound.

FIG. 29 is an overall system diagram depicting a method for generatingsuperimposed waves.

Elements and acts in the figures are illustrated for simplicity and havenot necessarily been rendered according to any particular sequence orexample.

DETAILED DESCRIPTION

In the following description, and for the purposes of explanation,numerous specific details, process durations, and/or specific formulavalues are set forth in order to provide a thorough understanding of thevarious aspects of exemplary examples. It will be understood, however,by those skilled in the relevant arts, that the apparatus, systems, andmethods herein may be practiced without these specific details, processdurations, and/or specific formula values. It is to be understood thatother examples may be utilized and structural and functional changes maybe made without departing from the scope of the apparatus, systems, andmethods herein. In other instances, known structures and devices areshown or discussed more generally in order to avoid obscuring theexemplary examples. In many cases, a description of the operation issufficient to enable one to implement the various forms, particularlywhen the operation is to be implemented in software. It should be notedthat there are many different and alternative configurations, devices,and technologies to which the disclosed examples may be applied. Thefull scope of the examples is not limited to the examples that aredescribed below.

In the following examples of the illustrated examples, references aremade to the accompanying drawings which form a part hereof, and in whichis shown by way of illustration various examples in which the inventionmay be practiced. It is to be understood that other examples may beutilized and structural and functional changes may be made withoutdeparting from the scope of the invention.

So as to reduce the complexity and length of the Detailed Specification,and to establish the state of the art in certain areas of technology,Applicant(s) herein expressly incorporate(s) by reference all of thefollowing materials identified in each numbered paragraph below. Theincorporated materials are not necessarily “prior art”.

U.S. Provisional Patent Application No. 62/199,098 to Preston, et al,filed Jul. 30, 2015, entitled: System and Method for Fiber OpticSensing, herein incorporated by reference in its entirety.

“Adelos.3.r.9.6” (29 pages) by Dan Preston dated June 2015, herebyincorporated by reference in its entirety and included as non-patentliterature on the submitted information disclosure statement of Aug. 27,2015 due to minimal publication data of the proprietary document.

“Adelos 1.1 FPGA Architecture Rev 0.6” (43 pages) by John Providenzadated Oct. 30, 2009, hereby incorporated by reference in its entiretyand included as non-patent literature on the submitted informationdisclosure statement of Aug. 27, 2015 due to minimal publication data ofthe proprietary document.

“Adelos S4 Training Guide.Ph.2 (v1)” (77 pages) by Earonoff dated Aug.3, 2011, hereby incorporated by reference in its entirety and includedas non-patent literature on the submitted information disclosurestatement of Aug. 27, 2015 due to minimal publication data of theproprietary document.

“Adelos Firmware Coding Complete” (82 pages), hereby incorporated byreference in its entirety and included as non-patent literature on thesubmitted information disclosure statement of Aug. 27, 2015 due tominimal publication data of the proprietary document.

“Adelos Hardware Coding Complete” (438 pages), hereby incorporated byreference in its entirety and included as non-patent literature on thesubmitted information disclosure statement of Aug. 27, 2015 due tominimal publication data of the proprietary document.

“Adelos Report Software” (18 pages) by Tim Roberts dated Mar. 9, 2010,hereby incorporated by reference in its entirety and included asnon-patent literature on the submitted information disclosure statementof Aug. 27, 2015 due to minimal publication data of the proprietarydocument.

“Adelos Sensor System” (16 pages) by Providenza & Boekelheide, Inc.dated Nov. 19, 2013, hereby incorporated by reference in its entiretyand included as non-patent literature on the submitted informationdisclosure statement of Aug. 27, 2015 due to minimal publication data ofthe proprietary document.

“Adelos Software Coding Complete” (632 pages), hereby incorporated byreference in its entirety and included as non-patent literature on thesubmitted information disclosure statement of Aug. 27, 2015 due tominimal publication data of the proprietary document.

“Adelos 2.0 FPGA Architecture v0.1” (36 pages) by John Providenza datedAug. 6, 2013, hereby incorporated by reference in its entirety andincluded as non-patent literature on the submitted informationdisclosure statement of Aug. 27, 2015 due to minimal publication data ofthe proprietary document.

PCT Patent Application No. PCT/US1997/009892 to Bridge et al, filed Jun.6, 1997, entitled: Retroflectively Reducing Coherence Noise inReflectometers, herein incorporated by reference in its entirety.

“Fiber Couplers” RP Photonics Encyclopedia.http://www.rp-photonics.com/fiber_couplers.html. 3 pages

U.S. Pat. No. 6,043,921 to Payton, issued Mar. 28, 2000, entitled:Fading-Free Optical Phase Rate Receiver, herein incorporated byreference in its entirety.

U.S. Pat. No. 7,030,971 to Payton, issued Apr. 18, 2006, entitled:Natural Fiber Span Reflectometer Providing a Virtual Signal SensingArray Capability, herein incorporated by reference in its entirety.

U.S. Pat. No. 7,268,863 to Payton, issued Sep. 11, 2007, entitled:Natural Fiber Span Reflectometer Providing a Spread Spectrum VirtualSensing Array Capability, herein incorporated by reference in itsentirety.

U.S. Pat. No. 7,271,884 to Payton, issued Sep. 18, 2007, entitled:Natural Fiber Span Reflectometer Providing a Virtual Phase SignalSensing Array Capability, herein incorporated by reference in itsentirety.

U.S. Pat. No. 7,274,441 to Payton, issued Sep. 25, 2007, entitled:Natural Fiber Span Reflectometer Providing a Virtual Differential SignalSensing Array Capability, herein incorporated by reference in itsentirety.

U.S. patent application Ser. No. 10/776,832 to Evans et al, filed Feb.11, 2004, entitled: Active Fiber Loss Monitor and Method, hereinincorporated by reference in its entirety.

U.S. patent application Ser. No. 10/711,918 to Tarvin et al, filed Oct.13, 2004, entitled: System and Method to Interpret DistributedTemperature Sensor Data and to Determine a Flow Rate in a Well, hereinincorporated by reference in its entirety.

U.S. patent application Ser. No. 13/221,280 to Hartog et al, filed Aug.30, 2011, entitled: Distributed Fiber Optic Sensor System with ImprovedLinearity, herein incorporated by reference in its entirety.

U.S. patent application Ser. No. 13/751,054 to Skinner, filed Jan. 26,2013, entitled: Distributed Acoustic Sensing with Multimode Fiber,herein incorporated by reference in its entirety.

Applicant(s) believe(s) that the material incorporated above is“non-essential” in accordance with 37 CFR 1.57, because it is referredto for purposes of indicating the background of the invention orillustrating the state of the art. However, if the Examiner believesthat any of the above-incorporated material constitutes “essentialmaterial” within the meaning of 37 CFR 1.57(c)(1)-(3), applicant(s) willamend the specification to expressly recite the essential material thatis incorporated by reference as allowed by the applicable rules.

Technologies are described herein for a fiber-optic sensor system thatdetects perturbations or pressure strain variation in a fiber opticcable by measuring changes in reflected laser light. The system isdirected at processing telemetry in real-time, recording telemetry datafor later playback and analysis, and presenting waterfall displays andaudio output for real-time monitoring of threats and situational status.Longer lengths of sensing fiber may be used depending on parameters andsensing methods.

Glossary

There are a number of terms in this document that have unique meaningsin the context of this disclosure:

CW—Continuous Wave. A continuous wave is an electromagnetic wave ofconstant or near constant amplitude and frequency; and in mathematicalanalysis, of infinite duration.

DAS—Distributed Acoustic Sensing. In DAS, the optical fiber cablebecomes the sensing element and measurements are made, and in partprocessed, using an attached optoelectronic device. Such a system allowsacoustic frequency strain signals to be detected over large distancesand in harsh environments.

DTS—Distributed Temperature Sensing. DTS are optoelectronic deviceswhich measure temperatures by means of optical fibers functioning aslinear sensors. Temperatures are recorded along the optical sensorcable, thus not at points, but as a continuous profile. A high accuracyof temperature determination is achieved over great distances. Typicallythe DTS systems can locate the temperature to a spatial resolution of 1m with accuracy to within ±1° C. at a resolution of 0.01° C. Measurementdistances of greater than 30 km can be monitored and some specializedsystems can provide even tighter spatial resolutions.

DTSS—Distributed Temperature and Strain Sensing.

MMF—Multimode Fiber. The primary difference between multimode and singlemode optical fiber is that multimode has much larger core diameter,typically 50-100 micrometers; much larger than the wavelength of thelight carried in it. Multimode fiber supports more than one propagationmode which limits the fiber by modal dispersion. Due to the modaldispersion, multimode fiber has higher pulse spreading rates than singlemode fiber, limiting multimode fiber's information transmissioncapacity. Single mode fibers are most often used in high-precisionsensing applications because the allowance of only one propagation modeof the light makes the light source easier to focus properly.

OTDR—Optical Time-Domain Reflectometer. An optical time-domainreflectometer is an optoelectronic instrument used to characterize anoptical fiber. An OTDR is the optical equivalent of an electronic timedomain reflectometer. It injects a series of optical pulses into thefiber under test. It also extracts, from the same end of the fiber,light that is scattered (Rayleigh backscatter) or reflected back frompoints along the fiber. The strength of the return pulses is measuredand integrated as a function of time, and plotted as a function of fiberlength.

PRC—Pseudo-Random Code. A sequence of reproducible random pulses,produced by a polynomial. A PRC correlates very well with itself, butvery poorly when one of the signals being correlated is delayed. The useof a PRC allows one to pick out a particular transmitter when a largenumber of transmitters are sending the same sequence at different times.

RF—Radio Frequency. Radio frequency is a rate of oscillation in therange of around 3 kHz to 300 GHz, which corresponds to the frequency ofradio waves, and the alternating currents which carry radio signals. RFusually refers to electrical rather than mechanical oscillations;however, mechanical RF systems do exist

ROS—Rayleigh Optical Scattering. Rayleigh scattering is the (dominantly)elastic scattering of light or other electromagnetic radiation byparticles much smaller than the wavelength of the light. The particlesmay be individual atoms or molecules. Rayleigh scattering results fromthe electric polarizability of the particles. The oscillating electricfield of a light wave acts on the charges within a particle, causingthem to move at the same frequency. The particle therefore becomes asmall radiating dipole whose radiation can be seen as scattered light.

ROSE—Rayleigh Optical Scattering and Encoding.

Sample—The telemetry readings from one point in time. In someconfigurations, a sample contains 4,096 16-bit floating pointnumbers—one for each zone, for each polarization, for each quadraturephase. The Digital Signal Processor (DSP) refers to this as a TelemetryProcessing Unit (TPU).

SMF—Single Mode Fiber. SMF is designed to carry light only directly downthe fiber—the transverse mode. Modes are the possible solutions of theHelmholtz equation for waves, which is obtained by combining Maxwell'sequations and the boundary conditions. These modes define the way thewave travels through space, i.e. how the wave is distributed in space.Waves can have the same mode but have different frequencies. This is thecase in single-mode fibers where waves can have the same mode butdifferent frequencies which means that they are distributed in space inthe same way, and provide a single ray of light. Although the raytravels parallel to the length of the fiber, it is often calledtransverse mode since its electromagnetic vibrations occur perpendicular(transverse) to the length of the fiber.

P and S refer to two polarizations of the laser light and are explainedfurther in a later section.

An understanding of three phenomena—two physical (Rayleighbackscattering and fiber stretching), and one mathematical(pseudo-random code) are helpful in understanding the presentdisclosure.

Rayleigh Backscattering

The laser light source is modulated by injecting a known, repeatingpattern. This modulated signal is reflected back to the light origin byRayleigh backscatter all along the fiber optic cable. Light reflectedfrom a given point will return to the source, with a delay based on thespeed of light in the fiber. Assuming the speed of light in the fiber isabout 200,000,000 m/s, it will take 100 ns for the signal to travel out10 meters and reflect back through 10 meters. If the signal is preciselycorrelated 100 ns after it was transmitted, it will be found within thelarge number of reflections coming back from the fiber length.

For purposes of explanation, the speed of light in a vacuum is known tobe 299,792,458 m/s. Light in a fiber is slowed based on the refractiveindex of the fiber. At the 1319 nm wavelength produced by the laser, theSMF-28e fiber currently used in some example configurations has arefractive index of 1.4677. That makes the speed of light within thefiber 204,260,038 m/s. For discussion purposes, it will be rounded to200,000,000 m/s, but in the field it is necessary to remember that thisestimate is 2.13% low. When it is said that a zone is 1 meter, thephysical reality is that a zone is 1.0213 m. The difference is notimportant for discussion, but may be important in operation. Theanalysis software takes this into account when displaying distances.

With the above in mind, Continuous Wave lasers have a distinct advantageover pulsed laser. Pulse modulating a CW laser is not the same as pulsedlaser. It is both well understood in the art and to some extentmisunderstood. Many inventions in the art of interferometry will falselylabel and describe pulse modulated in an effort to traverse certainprior art related to CW. The reality is at long ranges, the best apulsed laser will achieve is 1KHz sampling, where CW will allow for muchhigher rates, e.g. 24KHz. The following discussion describes thelimitations of pulsed laser; all values approximate.

Assume a 50 KM cable length and fiber optics roughly ⅓ slower; what isthe maximum sampling rate achievable with a pulsed laser, anticipateround trip delay:

-   -   Light travels in a vacuum roughly 0.3 M/1(10⁻⁹) Seconds    -   Assume a 50 KM Cable with a 100 KM round trip    -   Since fiber is a third slower, 0.3 M/1(10⁻⁹) Seconds*.67        =0.21M/1(10⁻⁹) Seconds    -   1 light pulse will take [100,000/0.2 M/1(10⁻⁹) Seconds] or        roughly 5(10⁻⁴) Seconds per pulse    -   Dividing now 1 second/5.0 (10⁻⁴) Seconds per pulse yields a max        frequency of roughly 2 kHz with a Nyquist Frequency of 1 kHz.    -   Shorter distances obviously yield higher sampling rates

As long as the PRN code is not repeated, and coherent signals can beretrieved, sampling can be performed at much higher hates (fidelity) andmuch longer distances. Also, a second consideration is spatialresolution which is mainly determined by the duration of the transmittedpulse, with a 100 ns pulse giving 10 m resolution being a typical value.The amount of reflected light is proportional to the pulse length sothere is a trade-off between spatial resolution and maximum range. Toimprove the maximum range, it would be desirable to use a longer pulselength to increase the reflected light level but this leads to a largerspatial resolution. In order for two signals to be independent, theymust be obtained from two points on the fiber that is separated by atleast the spatial resolution. It is possible to obtain samples atseparations less than the spatial resolution and although this producessignals that are not independent of each other, such an approach doesoffer advantages in some applications. The separation between thesampling points is sometimes referred to as the spatial sampling period.

Fiber optic cables are not perfect. They contain a huge number of verytiny imperfections. Those imperfections reflect a small fraction of thelight being transmitted through the cable. This reflected light can bemeasured back at the cable origin source.

Fiber Stretching

The cable sensitivity of fiber affects disturbances detected. Anydisturbance near the cable, for instance, buried in the ground, such asfootsteps, vehicles, rock falls, voices, etc., sends a small shockwaveor pressure wave through the ground. Those small shockwaves disturb thefiber, causing the fiber to stretch microscopically. Thosemicro-stretches cause the light signal to be delayed slightly, e.g., aphase shift. This delay changes the success of the attempt to correlatethe signal at precise delay points. By measuring the changes incorrelation, the frequency of the disturbance that impinged on the cablecan be determined. The pressure wave impact on the buried fiber opticcable can be referred to as “coupling effect,” the physical mechanism ofhow pressure is transmitted through a medium like soil against the fibercoating. Enhancing and maximizing the coupling is a key to measuringsuccessfully the change in the arrival and departure of light throughmicro-strains in the fiber optic cable.

The Rayleigh backscatter reflections are at a very low level. Tooptimize the correlation opportunities, the modulated signal is read attwo different polarizations, labeled S and P. The laser is polarized inone direction, but the fiber randomizes the polarization to a certaindegree. When one polarization fades away because of conditions in thefiber, the other polarization will tend to rise.

Pseudo-Random Code

A mathematical phenomenon helps to make it possible to use a standardfiber and a standard continuous wave (CW) laser. In some exampleconfigurations, the hardware generates a non-repeating pseudo-randomcode (PRC) sequence which is modulated onto the laser at a 100 MHzsymbol rate. One aspect of the PRC sequence is that it has veryimportant auto-correlation properties. A code will correlate extremelywell with itself if it is exactly phase aligned. If it is poorlyaligned, it correlates very poorly.

This is the fundamental principle behind the correlators. As an example:at 100 MHz, the PRC units are sent once each 10 ns. In 10 ns, laserlight in the fiber travels approximately two meters—one meter out, andone meter back. Thus, a correlation unit can “look for” a time delayedversion of the code that represents a specific section of the fiber. Bycorrelating against the PRC sequence delayed by ten cycles, thecorrelation unit will get its best match to signals from ten meters downthe fiber, and will tend to reject all of the other reflections.

The values used in this description serve as an example. It should beunderstood that other values may be used depending upon the sensingmethods, equipment, system requirements, preferences, and othervariables within each system. There are many parameters and sensingmethods that can be used in different configurations to meet differentrequirements. System Operation

The Fiber Optic Interrogator and Data Logger, depicted in FIG. 1 andreferred to herein as the base configuration, comprises a highlyintegrated and optimized fiber optic interrogator package (integratedoptics) 500, embedded controller 510, mass storage 520 of raw data andtiming reference, large bandwidth Ethernet for data transfer, controlpanel 515 software with Ethernet link to the embedded controller 510,and fiber under test 505. In an example, all optical components areoptimized in a standalone package based on a JDSU continuous wave (CW)laser. The integrated optical system 500 may include a built-in powersupply 490. In some examples the integrated optics 500 may be 3Dprinted.

The embedded controller 510 sends operating control signals to the laser405. The laser 405 emits light as a continuous wave (CW) or a pulsemodulated signal into splitter E 410 which splits the signal into areference signal and an interrogation signal. The reference signal ismodulated by an acousto-optic modulator (AOM) 415 and the interrogationis modulated by an electro-optic modulator (EOM) 420. A portion of themodulated reference signal from the AOM 415 is transmitted to theintegrated and optimized mixer subsystem 525 to splitter C 450. Theremaining portion of the modulated reference signal from the AOM 415travels to photo diodes 470 and into amplifier 475. The modulatedinterrogation signal from the EOM 420 travels into a circulator 430. Thecirculator 430 transmits the modulated interrogation signal throughcoupler 435 and out into the fiber under test 505. A modulated signal isbackscattered from the fiber under test 505 back through coupler 435 andinto the circulator 430. The modulated signal backscattered from thefiber under test 505 travels through the circulator 430, into isolator425, then into the signal mixer subsystem 525 at splitter A 440. Thesignal mixer 525 comprises a plurality of signal splitters and or signalcombiners. Splitter A 440 splits the signal into combiner B 445 andcombiner D 455. The modulated reference signal from the AOM 415 enterssplitter C 450 which splits the signal into combiner B 445 and combinerD 455. Combiner B 445 transmits superimposed waves into photo diodes 460and then into amplifier 465. Combiner D 455 transmits superimposed wavesinto photo diodes 480 and then into amplifier 485. Amplifiers 465, 475,and 485 amplify the superimposed waves and transmit them into RF links540, 545, and 550, respectively which convert them to radio signals andtransmits the resultant RF signals to the embedded controller 510.

The embedded controller 510 further transmits control information to theAOM 415 and the EOM 420 through RF generators 530 and 535, respectively.Data is transmitted back and forth between the embedded controller 510and mass storage 520 as well as between the embedded controller 510 andthe control panel 515.

In some examples the fiber under test has a coating thereon made of athermoplastic material having the combined characteristics of a lowYoung's modulus and a Poisson's ratio below that of natural rubber,wherein the coating enhances the longitudinal component of strainvariation derived from an acoustic wave signal. The fiber under test hasa length L and the light source is a laser having the capability togenerate a signal with sufficient stability to retain coherency inpropagation along the fiber under test for a distance at least equal totwo times the length L. The fiber under test may be single mode,multimode, or polarization preserving fiber optic cable.

Referring to FIG. 1, the base configuration further comprises a VMEbus605. The VMEbus 605 is a non-proprietary computer bus standard thatfacilitates forward and backward compatibility and multi-processing(1-21 processors). The VMEbus uses asynchronous daisy chain,master/slave architecture. The VMEbus, well known in the art, comprisesa number of slots into which modular cards can be inserted. Each modularcard adds additional functionality to the embedded controller 510.

FIG. 2 depicts the base configuration of FIG. 1 equipped with an exampleassortment of modular cards. The cards shown are radio frequency (RF)mixing 700, signal correlation 800, control logic 900, acousticintensity 1000, event interrogation 1100, noise reduction 1200, up tocard N expansions. In the depicted example the modular cards arenumbered 1 to N, however, the purpose of the numbering is merely to aidin the description and does not necessarily reflect priority or order ofinstallation.

FIG. 3 depicts a first expansion card of FIG. 2—RF Mixing 700 and Analogto Digital Conversion (ADC), referred to herein as Card 1. The purposeof Card 1 700 is to retrieve the RF signals from the fiber under test505 and convert them into digital signals for further processing.

The RF signals enter Card 1 700 from RF links 530, 535, 540, 545, and550. The RF signals from RF links 530 and 535 transmit data from the AOMRF generator 760 and the EOM RF generator 770, respectively. The RFsignal from RF link 545 is the power feedback for laser control. It isamplified through amplifier 740 and passed to ADC 750.

Local oscillator 710 outputs a signal that is split by RF splitter 725and relayed into mixers 735 and 745. In an example, the local oscillator710 outputs a 900 MHz signal added to a 10 Hz beat frequency. The RFsignal from RF link 540 is amplified by amplifier 705, attenuated byattenuator 720, and relayed to mixer 735 where it is mixed with aportion of the signal from local oscillator 710. The signal from RF link550 is amplified by amplifier 715 attenuated by attenuator 730 andrelayed to mixer 745 where it is mixed with a portion of the signal fromlocal oscillator 710.

The mixers 735 and 745 output P and S signals to the ADC 750. Theresulting digital signal is sent from Card 1 700 to VME 605 and furtherforwarded to mass storage 520 and control panel 515. Further informationis passed to the VME 605 from the local oscillator 710 and theattenuators 720 and 730.

FIG. 4 depicts a second expansion card of FIG. 2—Signal Correlation 800,referred to herein as Card 2. Card 2 800 uses a digital signal processor(DSP) to take the converted signals from Card 1 700 and correlate theminto telemetry information.

To optimize the correlation opportunities, the modulated signal is readat two different polarizations, labeled S and P. The laser is polarizedin one direction, but the fiber randomizes the polarization to a certaindegree. When one polarization fades away because of conditions in thefiber, the other polarization will tend to rise.

The converted signal data is retrieved from memory 805 and passed intothe correlator system 810. The in-phase and quadrature phase S signals(IS and QS) are correlated in a first correlator, C1S, 830 andtransmitted to a second correlator, C2S, 835 then to telemetry 850. Thein-phase and quadrature phase P signals (IP and QP) are correlated in afirst correlator, C1P, 840 and transmitted to a second correlator, C2P,845 then to telemetry 850. The telemetry information is then transmittedto a communications manager 820. Card 2 800 may also include amicroprocessor 825 and a memory management unit (MMU) 815.

FIG. 5 depicts a third expansion card of FIG. 2—Control Logic 900 andData Logger, referred to herein as Card 3. Card 3 900 provides controllogic to the system components. Card 3 900 comprises data logging logic905, local oscillator (LO) control 910, EOM control 915, AOM control920, pseudo-random noise (PRN) generator 925, laser control 930, laserpower manager 935, programmable attenuators 940, modulator control 945,memory 950, communications manager 955, MMU 960, and microprocessor 965.

The data logger 905 provides the data logging logic including timestampsand multiplexing multiple signals IQ, IP, SQ, and SP (described furtherin FIGS. 6 and 7), into one signal and stores the information in binary.The local oscillator (LO) 910, EOM 915, and AOM 920 control logicprovides control data to the corresponding hardware components. Thepseudo-random noise (PRN) generator 925 provides a PRN code to the AOM.Laser control 930 and laser power manager 935 are used to monitor andcontrol the laser. Programmable attenuator 940 and modulator 945 controlthe corresponding hardware components. The memory 950 is flash memory.Data is stored in mass storage 520.

Cards 1 through 3 700, 800, and 900 are required for basic data loggingpurposes. Additional cards are required to process and classify thelogged data. Cards 1 through 3 700, 800, and 900 are not integrated intothe base system. Allowing them to be modular allows for scalingprocessing capabilities to project-specific requirements, simple systemupgrades, and rapid reconfiguration.

The beat signal produced by the demodulation causes the phase of thevector to rotate through 360 degrees. In an ideal system with noimpetus, the vector length would remain constant, describing a circle,as depicted in FIG. 6. This attribute is used to normalize the signalprocessing. Depending on the optional cards installed, this data may bemonitored on the user interface in the form of Lissajou curves.Assigning the phase data to Cartesian coordinates with the in-phase (I)value as the x-axis and the quadrature phase (Q) value as the y-axisallows for conversion of each correlation value to a vector using anarctangent. The change in the angle of that vector (ΔΦ) from sample tosample yields the relative change in correlation strength, phase, forthat particular zone. The result is the audio reading for the sample.The length of the vector indicates the power for the sample. Generally,the algorithms depicted in FIG. 6 are known in the art and are includedas illustrative examples.

FIG. 7 depicts a fourth expansion card of FIG. 2—Acoustic Intensity1000, referred to herein as Card 4. The purpose of Card 4 1000 is tomanage acoustic event intensity.

Telemetry data is retrieved from memory 1005 from one of the correlatorsor mass storage 520. The telemetry data, IP, QP, IS, and QS (furtherdefined in FIG. 10 description: IP and QP represent quadrature data, 90°out of phase, for the “parallel” polarization from the fiber and IS andQS represent quadrature data for the “perpendicular” polarization), ispassed through wild point smoothing 1030 to eliminate noise and fill inmissing values, providing a cleaner output signal.

Once the signal has been smoothed, the P signal data and the S signaldata proceed through separate circle corrections 1035 and 1040,respectively and then to vector 1045 and 1050, respectively. Change inphase (ΔΦ) data 1055 and power data 1060 is then merged from informationobtained from both vectors 1045 and 1050. The resulting power and ΔΦdata are the basis for the remainder of the signal processing. Change inphase data (ΔΦ) 1055 is transmitted to the VME 605.

Power data 1060 is transmitted via user datagram protocol (UDP) packetto power stream 1065 and finally to VME 605. Card 4 1000 may alsoinclude a microprocessor 1025, communications manager 1020, and a memorymanagement unit (MMU) 1015.

FIG. 8 depicts a fifth expansion card of FIG. 2—Event Interrogation 1100and Demodulation, referred to herein as Card 5. Card 5 1100 providesadditional functionality to Card 4 1000.

Change in phase data (ΔΦ) 1055 is retrieved from memory 1105 from one ofCard 4 1000 or from mass storage 520. The ΔΦ values from the twopolarizations are combined in proportion to the power readings. Theresulting power and ΔΦ are the basis for the remainder of the DSPprocessing, which produces a series of products on various UDP ports,for consumption by other applications. The change in phase data (ΔΦ)1055 is passed through low-pass filter 1135 to attenuate noise. In anexample, the low-pass filter 1135 attenuates outside the range of 18 Hzto 300 Hz. The filtered signal is sent via UDP packet to the audiostream 1145.

A Fast Fourier Transform (FFT) 1140 is then performed on the change inphase (ΔΦ) 1055 values. The power spectrum of the FFT 1140 is computedand the standard deviation of the power spectrum is compared to theaverage power spectrum over time. The standard deviation becomes thedetector data stream and can be used by other data analysis software asa first-order estimate of the level of activity in the correspondingzone. By monitoring the values over time and comparing the values toadjacent zones, analysis software can get a first indication that anevent has occurred. Card 5 1100 merely indicates that an event hasoccurred—additional cards are necessary to process and classify theevent. In an example, the FFT 1140 is run on 2,048 points. This numberof points processed by the FFT 1140 limits resolution for featurediscrimination in the detector stream. From the FFT 1140 the signal datais passed via UDP packet to the detect stream 1150. The resultinginformation is passed from the audio stream 1145 and the detect stream1150 to the VME 605. Card 5 1100 may also include a microprocessor 1125,communications manager 1120, and a memory management unit (MMU) 1115.

Audio data are produced by subtracting the ΔΦ values for any two zones.When the fiber stretches, the light is delayed from that point all theway down the fiber. Thus, the audio reading for the zones at the end ofthe fiber includes all of the stimuli that occurred on the entire lengthof the fiber. By subtracting the zone X value from the zone Y value, onegets only the stimuli that occurred between zone X and zone Y. In anexample, audio streams can be produced for two or more zone pairs at atime.

FIG. 9 depicts a sixth expansion card of FIG. 2—Noise Reduction 1200 andClassification, referred to herein as Card 6. The purpose of Card 6 1200is to reduce signal noise to provide cleaner signal output. Cleanersignal output allows for more accurate classification of events.

The signal is passed to Card 6 1200 from memory 1205 from one of Card 51100 and mass storage 520 to noise reduction architecture 1210 where itis processed and passed to the VME 605. Card 6 1200 may also include amicroprocessor 1225, communications manager 1220, and a memorymanagement unit (MMU) 1215.

An interrogation light wave source may be generated by modulating theamplitude, phase, or polarization of a coherent light wave with atime-structured correlation code. The correlation code can be a seriesof pulses, chirps, binary sequences, or any other type of code whichprovides the required correlation characteristics. Therefore, delayingthe correlation decoding/de-multiplexing function allows de-multiplexingof delay multiplexed signals identifiable by speed of propagation anddistance of flyback travel.

Referring now to the integrated optics 500 of FIG. 1 in more detail, alaser 405 launches an interrogation signal into fiber under test 505 andretrieves light wave back propagation from a continuum of locationsalong the fiber span 505. Back propagation mechanisms may includeRayleigh Optical Scattering (ROS) and other effects generated within theoptical fiber 505. ROS in an optical fiber 505 backscatters lightincident upon the fiber 505. The incident light travels down the opticalfiber 505 to the scattering point/region. At the scattering region theincident light is backscattered back up the optical fiber 505. As thelight travels the round trip optical path (i.e., distance of flybacktravel) any disturbances of the fiber 505 which increase or decrease theoptical path length will cause the phase of the incident andbackscattered light to be modulated. Suppose a pressure is applied tothe optical fiber 505. The pressure elongates the path length of thelight traveling through the region.

The backscattered wave arriving back at an optical coupler 435 from ROSEfiber optic array 505 passes into circulator 430. The backscatteredlight which arrives at circulator 430 is the summation of all lightbackscattered from a continuum of locations along the length of the ROSEfiber optic span 505.

Furthermore, the phase of the channel output at a first location will bethe summation or integration of all pressure changes along thebi-directional path. This unusual phenomenon has been demonstrated withexperimental hardware. Once the correlation process isolates the opticalsignal originating from a spatial region, the signal must be phasedemodulated to extract the pressure information.

The system also applies to point-wise non-distributed sensors orartificially generated multiplexing by electronics means. Theinterrogation light wave can be intercepted and retransmitted back tothe receiver with an artificial, electronically generated delay, as ameans of delay/correlation multiplexing many channels. Moreparticularly, the propagation of the optical spread-spectruminterrogation signal down the continuous full span of the optical fiberspan 505, signal launch end to remote end, causes a back-propagatingcomposite optical signal, which is the linear summation, or integrationspatially, of all of the individual, continuous, or continuum ofback-reflections along the span of the optical fiber 505.

One component of this composite signal is comprised of the naturallyoccurring continuum of optical back reflections (ROSE) of the opticalspread spectrum carrier signal that is formed by modulating the primarycarrier signal by the spectrum spreading signals. Another component iscomprised of the artificially occurring optical back reflections,either-point wise reflections or distributed reflections, of the opticalspread spectrum carrier signal that is formed due to propagationdiscontinuities as the result of presence of a fiber cable coupler 435in span 505. Still another component comprised of the continuum ofmodulations at locations along the span of the reflected signals due tolongitudinal components of optical path length change, causing a delayin the reflected signal, experienced by the fiber optical span 505 alongits length.

Such optical path length change or delay may be caused by a variety ofpossible sources including acoustic pressure waves incident to the fiber505, electromagnetic fields coupled to the fiber 505, mechanical strainor pressure on the fiber 505, thermal strain or pressure induced in thefiber 505, or other means of causing change in the optical path length.Use of the acoustic pressure wave's mode of changing path length inperimeter intrusion monitoring systems is the principle exampleillustrated herein. In this use, optical fiber span 505 is employed toprovide an array of virtual geophones buried at a range of depthsbeneath the surface of the ground of about between six to eighteeninches, to sense motion of an object on the surface of the ground. Theacoustic pressure wave sensing mode is also useful to sense seismicsignals, as for example as linear arrays inserted into casing structuresof existing oil wells. Predetermined artificial pressure wave producingshocks are imparted into the ground, and the responses from the sensorare used to locate secondary oil deposits. The acoustic pressure wavesensing mode is further useful for employing span 505 as an array ofvirtual hydrophones, with the media which couples the signals to thehydrophones at least in part being the body of water in which the arrayis immersed. Such hydrophone arrays find use as naval undersea warfaretowed arrays, or towed geophysical exploration arrays. In the latter thearrays respond to artificially produced shocks of predeterminedcharacter and location induced in the body of water, and the response ofthe array to bottom return signals are used to locate ocean bottomgeophysical feature indicating likely presence of an oil deposit. Yetfurther, a sensing position on a fiber span 505 could be used to receiveas input microphonic signals suitably imparted to the region of thesensing position. The electromagnetic field sensing mode of fiber span505 could be used for monitoring electronic signals along atelecommunication cable's span to localize malfunctions. Responses offiber span 505 to mechanical, pressure or thermal strains can be used insystems for monitoring such strains.

An alternate example of fiber 505 is to provide fiber of a polarizationpreserving or single polarization, optical fiber. The polarizationpreserving fiber of this type holds the backscattering light in a narrowrange of polarization states so that a substantially single RF signalenters a single set of correlators, reducing the complexity of thesystem.

The correlation code generator creates a signal that has a broadbandwidth. The broadband nature of the correlation code is required toobtain the desired properties in the signals autocorrelation function.The calculation and definition of the autocorrelation function of anygeneral signal is well known and defined in signal processingliterature. The correlation code signal is structured such that itsautocorrelation function is highly peaked at zero delay, and is verysmall away from zero delay. This criterion is well known to those ofskill in the art and is the essence of why the correlation code has abroad bandwidth. Any signal that has the desired autocorrelationfunction properties can be used as the correlation code. There are manyreasons for choosing one correlation code over another: ease ofcreation; autocorrelation properties; cost of creation hardware; cost ofcorrelation hardware; and effectiveness in producing spread spectrumsignal effects. In some configurations, the correlation code can be abinary sequence with a desired trans-orthogonal autocorrelation property(sometimes called a pseudonoise sequence), a pseudorandom number (PRN)sequence with the desired autocorrelation property, chirps, or othertypes of signals which provide correlations code having predictablenon-repetitive behavior. The foregoing list of types of sequence signalswhich may be employed to modulate the carrier light wave signal includesboth “binary pseudonoise sequences” and “pseudorandom number (PRN)sequences.” For purposes of construction of this specification and theappended claims, these terms are employed as they are defined under thelistings “Pseudonoise (PN) sequence (communication satellite)” and“Pseudorandom number sequence” at pages 747 and 748 of the “IEEEStandard Dictionary of Electrical and Electronic Terms” (FourthEdition), which listings are herein incorporated by reference in theirentirety. Further for purposes of construction of this disclosure, it isdeemed that “binary pseudonoise sequence” is generic and “pseudorandomnumber sequence” is a species thereof. Still further for purposes ofconstruction of this disclosure, both terms are deemed to include analogsignal forms of sequences as well as digital signal forms.

The temporal length of the code sequence which is reiteratively producedby generator may be either less than the time period for propagation ofa light wave to the remote end of span and propagation back of abackscattering (i.e. distance of flyback travel), or greater than thistime period. It cannot be equal to this period.

Refer to FIG. 1. The local oscillator and composite light waves areinterfered on photo diodes 460, 470, and 480 producing an electronicsignal which electronically represents the heterodyned opticalinterference power between the two light waves. The resulting compositeradio frequency signal at outputs from the amps 465, 475, and 485represent electronically the composite light wave signal. The compositeelectronic receiver signal is passed to the correlator system (FIG. 4).The local oscillator light wave on optical path 13 is interfered withthe composite light wave on optical path 11. The interference power isphoto-detected in photo diodes 460, 470, and 480 by opticallyinterfering with the composite back propagating light wave on the localoscillator signal. As one of the components of this interfering action,there is produced a difference beat signal which is a composite radiofrequency representation of the composite light wave on optical path 11.

This interfering of the local oscillator 710 (FIG. 3) output light waveand the composite back-propagating CW light wave 11 provides thetranslation of signal 11 from the optical domain to a CW radio frequency(RF) composite difference beat signal output from the amps 465, 475, and485. This reduces the frequency of signal into an electronicallyprocessable signal frequency range. It is to be appreciated that the RFcomposite difference signal produce by this translation action includeshaving counterpart components of the aforesaid components of thecomposite back-propagating light wave signal, with the phase states ofthese counterpart RF domain signals the same as the phase states of thecorresponding components of the back-propagating light wave.

In some examples, more than one light source is used. The lasers are tohave sufficiently stringent high performance capability with respect toexactness of frequency to enable interference effects there between andheterodyne detection of acoustic perturbation signals incident to fiber505 to produce beat frequencies within the radio frequency (RF) range.Also in accordance with examples, lasers have stringent performancecriteria with respect to the phase stability, or coherence, of theirbeams. They are to be substantially coherent over at least a propagationpath distance substantially equal to twice the length, L, of sensingfiber 505.

Each of the programmably selectable pairs of differenced phase signalsform a signal which is spatially bounded within the region of the fiberbetween zones. The phase differencer therefore produces differentialphase outputs corresponding to a set of virtual sensors withprogrammable length and position.

Stated another way, each programmable selection of pairs of phasesignals forms a virtual spatial differential sensor which senses thedifference between the phases of the output of the photo diodes 460,470, and 480. Each output is an RF difference beat signal representativeof the aforesaid “still another” component of the compositeback-propagating CW light wave signal which passes from the launch endof fiber span 505 to directional coupler 435. These signals from eachpair therefore represent signals of virtual spatial differential sensorsalong fiber span 505. As a result of the choice of pairs beingselectively programmable these virtual sensors can be employed toimplement adaptive apertures in processing signal incident the fiberspan 505. This feature would be useful, for example, in enablingsecurity system operators to classify objects causing acoustic pressurewave signals incident up a fiber span 505 used as a perimeter intrusionmonitoring line.

Referring to an example of FIG. 4. The composite radio frequency signal,or RF composite reference beat signal, which electronically representsthe received time-delay multiplexed optical signal, or compositeback-propagation CW light wave is input into the correlator system 810.The composite radio frequency signal is n-way split with power splitterinto a plurality (which can be multiple thousands or more) of electronicpathways. The master correlation code is input into the correlatorsystem 810. The correlation code is distributed to such a plurality ofprogrammable delay circuits. Each programmable delay circuit delays themaster correlation code by the delay required to decode/de-multiplexeach time-delay multiplexed channel. The plurality of programmable delaycircuits output a plurality of delayed correlation codes. Each of theoutputs therefore produces the corresponding de-multiplexed signal whichis time-gated by the corresponding time-delay of the correlation code.

These spatial delays are based on the time of propagation for flybacktravel along these distances, which are arbitrary and programmable. Thetime delay multiplexing of the optical signals comprising the compositeback-propagating optical signal arise from a plurality of spatiallocations causing a like-plurality of time-delays. The correlator systemspatially separates the components of the RF composite difference beatsignal into channels which each uniquely represent an optical signal ata single spatial location. The correlator system allows the spatialsampling of the optical signals so that a virtual array can be formedalong the fiber span 505 on FIG. 4

As an alternative example to the viewpoint inferable from the precedingsequence discussing FIG. 4, integrated optics 500 may be considered aspartitioned into: (i) an optical network for illuminating an opticalfiber sensing span, or other light propagation medium sensing span, andretrieving back propagating portions of the illumination; and (ii) aphotoelectronic network for establishing virtual sensors atpredetermined locations along the span and picking up external physicalsignals incident to, or impinging upon, the sensors.

In general, the optical network for the illumination of and for theretrieval of back-propagation from fiber span 505 comprises transmitterlaser 405, directional optical coupler 435, and optical fiber, or otherlight propagation medium 505.

The photoelectronic network for establishing virtual sensors and pickingup signals therefrom generally comprises two subdivisions. Onesubdivision provides a cyclically reiterative autocorrelatable form ofmodulation of the light wave illuminating fiber span 505. Thismodulation is in the form reiterated sequences having autocorrelatableproperties. The other subdivision takes the retrieved back propagationand performs a heterodyning therewith to obtain an RF beat signal. Itthen picks up the signal from the virtual sensors by autocorrelation andfurther processes it into more useful forms.

In general, the subdivision providing the cyclical reiterativemodulation of sequences illuminating fiber span 505 comprises a mastercorrelation code generator (via one of its electrical pathway outputs)and electro-optical modulator 420.

According to an alternate example, the system elements which perform theautocorrelation enable providing an output in the form of an RFcounterpart of a light wave time-domain reflectometry output of signalsincident to the virtual sensors as light wave time domain reflectometryoutputs a CW light wave modulated by a continuously reiterated binarypseudorandom code sequence is launched into an end of a span 505 ofordinary optical fiber cable. Portions of the launched light wave backpropagate to the launch end from a continuum of locations along the span505 because of innate fiber properties including Rayleigh scattering.This is picked off the launch end and heterodyned to produce an RF beatsignal. The RF beat signal is processed by a plurality (which can bethousands) of correlator type binary pseudonoise code sequencedemodulators respectively operated in different delay time relationshipsto the timing base of the reiterated modulation sequences. The outputsof the demodulators provide RF time-domain reflectometry outputsrepresentative of signals (e.g., acoustic pressure waves) incident tovirtual sensors along the fiber at positions corresponding to thevarious time delay relationships.

According to an alternate example, the system elements performing theautocorrelation enable detection of unique spectral componentsrepresenting phase variations of external signals incident to thevirtual sensors. A CW light wave modulated by a continuously reiteratedpseudorandom code sequence is launched into an end of a span 505 ofordinary optical fiber cable. Portions of the launched light wave backpropagate to the launch end from a continuum of locations along the span505 because of innate fiber properties including Rayleigh scattering.This is picked off the launch end and heterodyned producing an RF beatsignal. The RF beat signal is processed by a plurality (which can bethousands) of correlator type pseudonoise code sequence demodulation andphase demodulator units, operated in different time delay relationshipsto the timing base of the reiterated modulation sequences. These unitsprovide outputs representative of phase variations in respective uniquespectral components in the RF beat signal caused by acoustic signals, orother forms of signals, incident to virtual sensors at fiber positionscorresponding to the various time delay relationships.

According to an alternate example, a pair of the different delay timerelationships of the autocorrelation system elements are effective toestablish a virtual increment of the optical fiber span, and that asubstractor (where the substractor is a circuit that is capable ofsubtracting numbers, in particular, binary) circuit of a phasedifferencer (where the differencer is used to determine differencesbetween the signals) enables representing the differential phase signalacross the virtual increment. A CW light wave modulated by acontinuously reiterated pseudorandom code (PRC) sequence is launchedinto an end of a span of ordinary optical fiber cable. Portions of thelaunched light wave back propagate to the launch end from a continuum oflocations along the span because of innate fiber properties includingRayleigh scattering. This is picked off the launch end and heterodynedproducing an RF beat signal. The RF beat signal is processed by aplurality (which can be thousands) of correlator pseudonoise codesequence demodulation and phase demodulator units operated in differentdelay time relationships to the timing base of the reiterated modulationsequences. Pairs of outputs of the units are connected to respectivesubstractor circuits, each providing a signal representative of phasedifferential of incident acoustic signals, or other forms of signals,across virtual increments of the span established by a pair of saiddelay time relationships.

Example configurations enable the interrogation of ROSE fiber opticsensors, and the spatial sorting and separation of the temporal opticalphases of backscattered optical signals arising from a plurality ofvirtual optical sensors along fibers or other optical mediums. Examplesalso enable the spatial decoding of backscattered optical signals with abandwidth of tens of kilohertz. Examples also enable the sensorlocations along the fiber to be programmable, and allow the electronicseparation or segmentation of the array of fiber sensors intoprogrammable bounded lengths and positions. Because the correlationsignal can be designed to be a continuous wave, example configurationsincrease the average optical power considerably over conventional pulsedoptical phase sensor interrogation methods. Because the correlationsignal can be chosen to have spectrum spreading properties for whichdespreading (where despreading means to recombine signals that have beenspread or distributed) electronic circuitry is readily available,optical fiber system noise, such as reflection discontinuity noise dueto cable couplings, can be materially attenuated.

According to some configurations, a new capability of heterodyne opticalphase detection without resorting to dithered phase carrier methods isdisclosed. The phase demodulation method introduces heterodyne I & Qdemodulation to produce cosine and sine phase components, clipped signalamplitude stabilization techniques and digital signal processing basedphase detection. The spatially differential phase detection methoddescribed enables the rejection of unwanted lead-in fiber phase signals.

Distributed acoustic sensing using multimode fibers is performed usingessentially the same methods as with single mode fibers. Narrowbandpulses of laser light are generated in an optical source and the lightis launched through a beam splitter or coupler into the sensing fiber.In the multimode sensing fiber, the light pulses undergo coherentRayleigh backscattering and are reflected back, toward the opticalsource. When the backscattered pulses reach the beam splitter orcoupler, they are directed towards one or more photodetectors via amultimode return fiber rather than being allowed to return to the lightsource. Light leaving a fiber, either single mode or multimode, exits ina characteristic cone related to the fiber's numerical aperture. Anoptional lens may be positioned between the terminal end of multimodereturn fiber and the photodetector(s) in order to better control thespreading of the beam as it exits fiber. Data signals from thephotodetector(s) is fed via amplifiers and data acquisition modules to asignal processing module that performs DAS signal analysis usingtechniques known in the art.

The distance between the photodetector(s) and the terminal end of fiberor lens is predetermined by testing such that only one or a few modesare detected. Limiting the number of modes received by thephotodetector(s) improves the contrast of the interference signalsproduced by coherent Rayleigh scattering and makes possible the use ofmultimode optical fibers in DAS. In an alternate example the distancemay be either automatically or manually adjustable to achieve optimumperformance.

DAS signals are notorious for optical fading, where the signal strengthis time dependent due to slowly varying changes in the fiber that resultin changes in the optical path length between the interfering Rayleighscatterers in the fiber. By using a tested and preselected distance inconjunction with the photodetector(s), and thereby detecting one or asmall number of modes, the system can be optimized for thephotodetector(s) to generate the best signal for DAS analysis. Multipledetectors can be used to ensure that good quality signals are receivedalong the entire fiber by using a combination of detectors thatindividually measure good quality signals only at limited locationsalong the sensing fiber. Multiple detectors, each responding to a singlemode or a few modes, can eliminate signal fading.

Example for Laser Stabilization

As background it is not understood well in the art of interferometry theimportance of keeping the temperature and power output of the laserlight source stable. As laser temperature decreases, laser power outputincreases. As power output increases, signal strength increases.Increasing signal strength comes with an increase in background noise.As the laser power output is stabilized, the noise will also stabilize.If the noise is stable over time, it can be modeled and removed duringsignal processing. If the noise deviates from the noise removal model,i.e. due an unanticipated change in laser power and/or temperature, thenoise model will no longer fit actual noise and additional backgroundnoise will be present in the output signal. In an exemplary example thetarget laser power output is between 210 mW to 220 mW, however, otherranges are possible.

The laser and electronics may need to warm up for 15 to 30 minutes inorder to achieve proper operation before reliable data from the sensorarray can be gathered. The amount of time it takes to warm up is largelydependent on ambient air temperature. Generally, if the ambient airtemperature is below 40° C. recommended warm-up time is about 30minutes. For ambient air temperature above 50° C. the recommendedwarm-up time is generally about 15 minutes. Optimal operatingtemperature for laser electronics is generally around room temperature(21° C.). It should be noted that these values are merelygeneralizations as they are dependent upon the equipment used in thesystem.

The actual laser power will fluctuate when the system is first poweredup until the temperature stabilizes. In an exemplary example, the laserpower will fluctuate in the range of 215 mW to 235 mW when it is firstpowered up. This is normal, as the laser controller maintains the properlaser wavelength. The laser of this example is power adjustable. In theexample, the laser power output may be adjusted manually and/orautomatically.

In the base mechanical system the laser temperature in the optical fibersensing system needs to be kept within a specified range (dependent uponlaser type and model) to prevent variations to the output power whichcould increase background noise. In one example, depicted in FIG. 10,laser settings controls are mechanical. This example may comprise alaser 405, a heat sink 555, a cartridge heater 565, and a thermal cutout560. The laser 405 of the example has control electronics built into thelaser head including temperature and power output control. In thisexample, a target temperature or power can be input into the lasercontrol system and the laser control system itself will managetemperature or power and attempt to keep it within a predetermined range(specified by user and/or range specific to laser product capabilitiesand application). In an exemplary example a JDSU NPRO 126 laser is used;however, other lasers may be used. The laser 405 may be coupled to aheat sink 555. To compensate for a cold start, the laser 405 will notturn on until a specified base temperature is reached on the heat sink555. When the heat sink 555 reaches the specified temperature, theheater 565 will switch off to avoid overheating. In this example, theheater 565 is controlled with a thermal cutout 560. In some examples anadditional thermal cutout may be incorporated that is set to open up atan upper limit. The additional thermal cutout would be set to the uppertemperature limit of the installed laser 405.

FIG. 11 depicts the heater control circuit. The thermal cutout 560closes the heater control circuit when the temperature is below apredefined threshold. When the heater control circuit is closed, theheater 565 will turn on. In an exemplary example the predefinedthreshold is 10° C. When the thermal cutout 560 is closed, relay R1 1500is opened and the laser interlock is disabled. When the temperature iswithin the normal range (as specified by system components), i.e. normaloperation, the thermal cutout 560 will be open and thus the heatercontrol circuit will be open.

FIG. 12 depicts the laser interlock control circuit. The panel switch 11510, panel switch 2 1520, and key switch 1530 are all normally open.The laser interlock control circuit will only allow laser operation whenall three switches are closed and relay R1 1500 (FIG. 11) is notenergized. The normally closed (N.C.) contacts of relay R1 1540 openwhen the thermal cutout 560 (FIG. 11) closes the heater control circuitand the heater 565 (FIG. 10) is on. Relay R1 1500 (FIG. 11) is inparallel with the heater 565 (FIG. 10) and thus will energize when thethermal cutout 560 is closed, regardless of heater 565 (FIG. 10)operation. In examples with an upper limit thermal cutout, the cutoutwould be placed between normally closed (N.C.) contacts of relay R1 1540and key switch 1530 to open the interlock circuit as a safeguard againstand overheating of the laser.

To fulfill safety requirements, a key switch 1530 is implemented thatcan disable laser operation to keep light from being launched down thefiber during maintenance, etc. The laser enclosure is a sealed box withtwo normally open pressure switches (1510 and 1520) that are closed whenthe enclosure cover is in place. If the cover is removed, the switches(1510 and 1520) open and disable the laser 405.

Referring again to FIG. 10, the light from the laser 405 is launchedinto a splitter E 410. The splitter E 410 may split the power in half orin different percentages. In some examples, a larger portion of thelaser output power splits off to an electro-optical modulator (EOM) 420where it is modulated before continuing on to interrogate the sensingfiber 505. The remaining portion of the output power is split off to anacousto-optical modulator (AOM) 415 where it is again split. A portioncontinues from the AOM 415 to carrier and fading mitigation. Fadingmitigation occurs in the mixer 525 circuit of the integrated optics 500(FIG. 1). The remaining portion travels to the photo diodes 460, 470,and 480. In the depicted example, the photo diodes 460, 470, and 480together comprise five ports. Four of the ports are used for thequadrature information where IP and QP represent quadrature data, 90°out of phase, for the “parallel” polarization from the fiber and IS andQS represent quadrature data for the “perpendicular” polarization. Thefifth port is a test port which receives power feedback 495 from the AOM415. The signal then travels through an amplifier 465, 475, and 485where it is amplified and sent on to an opti-coupler which activates the“Laser Active” indicators.

The Mechanical System with the Microprocessor

In one or more examples a microprocessor 595 may replace or augment thethermal cutout 560. In an example of FIG. 13, where the microprocessor595 augments the thermal cutout 560, the microprocessor 595 will usesystem data to control operation of the thermal cutout 560. In someexamples the thermal cutout 560 may be replaced by a Variac styleswitch, wherein the temperature may be variably controlled. This exampleallows for more precise control of laser temperature and thereforeprovides better laser stabilization. In examples where themicroprocessor 595 replaces the thermal cutout 560, the microprocessor595 will control heater 565 operation directly. In some examples, themicroprocessor 595 may control the heater 565 directly, and a thermalcutout 560 may remain in the system as a redundant safety mechanism toprevent the laser 405 from becoming too cold.

In another example, a microprocessor 595 and feedback loop as depictedin FIG. 14, both laser temperature and laser output power may bemonitored and the data may be fed back to a microprocessor 595. Themicroprocessor 595 will dynamically control the laser temperature basedon system data. The laser power may be monitored at the fifth channel(power feedback 495) of the photo diodes 470 and the data may be sent tothe microprocessor 595 continuously in real time, or periodicallyaccording to user preferences and/or system processing power. The laserpower data may be used to ensure that the laser is working at optimalpower output. Any fluctuations in output power will cause themicroprocessor 595 to react to stabilize the power output. Themicroprocessor 595 receives temperature information 585 from one or moresensors located in proximity to the laser 405. The microprocessor 595receives laser power information from power feedback channel 495 anduses it to adjust the laser power 570. The microprocessor 595 controlslaser operations through laser control 575.

In another example, systems and methods are disclosed for a hardwarecontrol panel. In some examples, the hardware containment mechanism(s),including a box or boxes, may comprise one or more hardware controlpanels. In some examples the hardware control panel(s) comprises atleast one of: one or more hard switches such as on/off for one or morecomponents, a total reset button, fan speed controls, etc. In someexamples, all functionality provided on the hardware control panel(s) ispresent in the software. In some examples the hardware control panel(s)serves as physical control backups to operate or shut down systemhardware in the event of software failure. In some examples the hardwarecontrol panel(s) may further comprise one or more visual displaysindicating power levels, temperatures, fan speeds, etc. The softwarecontrol panel includes a diagnostics tab that can be used to check thehardware for any potential issues. The hardware containers (which may beone or more boxes or other containment systems) may include one or moremechanisms to reduce system noise such as sound dampening, vibrationdampening, and drop protection.

In some examples, a known strain may be induced on the fiber in order tochange the refractive index in a known manner. This allows more accuratereadings of disturbances in the vicinity of the fiber. Also, differentzones may have different noise floors depending on the environment ofthe area surrounding them (for instance, one area may be near a riverwhich would increase the noise floor).

In some examples, systems and methods are used for various couplingtechniques designed to elevate the detection threshold of fiber opticbased sensing systems. Techniques include fiber optic cable beingpositioned within a pipe and coupled with a filler material, layingfibers parallel with different depth relationships, laying fiber in atriangular wave pattern, placing a reflector beneath the plain fiber orpipe test section such that the signal is reflected back up as an echo,increasing the soil hydration, overlapping fiber in a gridconfiguration, and separation of soil regions by bulkheads. Theseconfigurations provide elevated detection thresholds useful for multiplepurposes such as predicting future movement and directionality. Notethat in this specification the term “section” refers to segments offiber and the term “region” refers to an area of soil or other materialsurrounding the fiber.

Referring to FIGS. 15A through 15E, the fiber optic cable may bepositioned within a pipe and surrounded by a coupling material whereinthe coupling material has increased acoustic sensitivity. Fiber within apipe could also be used as a test apparatus to experiment with differentcoupling materials in order to achieve the best results for increasedsensitivity for different environments. An example of the pipeconfiguration is depicted in FIG. 15A where the section of FIG. 15A isdesigned such that the coupling material may be evacuated and replacedto allow for testing of multiple different coupling materials. In anexample of FIG. 15A there is at least one tee connector with a verticalsection of pipe extending to the ground surface in order to release airpressure during fill and to provide visual indicator of fill levelduring filling, or level during evacuation, herein referred tocollectively as a t-valve 2020. A valve will be attached to eachvertical pipe section and can be shut once filling is complete toprevent contamination of the coupling material. The tee connector 2020may be placed near the center of the pipe section or it may be placed atthe highest altitude of the pipe section. A pump may be placed at thetee 2020 in order to pump the coupling material into the pipe 2025 aswell as to evacuate coupling material when replacing it. In the depictedexample vaults 2005 are placed at each end of the pipe section toprovide access to the fiber 2015. Optionally, the fiber 2015 may bespliced 2010 just before and just after the pipe section to simplifyremoval/replacement of the various components.

FIGS. 15B and 15C depict an example of FIG. 15A further comprising adrain 2030 at one end which may be used to remove the discarded couplingmaterial to a remote catch basin. The purpose of the drain 2030 istwofold: to keep the discarded coupling material from affecting the soilcharacteristics in the region surrounding the fiber 2025 and to keeppotentially damaging runoff (from the couplants) from affecting theenvironment. The catch basin is designed such that it can handle thecapacity of the coupling material and it can be easily removed for wastedumping.

FIGS. 15D and 15E depict an example of FIG. 15A further comprisingadditional t-valves 2020 for further pressure relief during filling andevacuation of the pipe 2025. FIG. 15D depicts an additional t-valve 2020at each end of the pipe section. FIG. 15E depicts additional t-valves2020 at intervals in the pipe section. Each t-valve 2020 should beplaced such that the valve is accessible. Placement may includeadditional vaults 2005 (not depicted) around each t-valve 2020.

FIG. 16 depicts placement of fiber at different depth relationships.FIG. 17 depicts a top view of the system discussed in FIG. 16. Thehatched portion in the drawing represents soil. The top hatched portionof the figure depicts the fiber placed deeper in fiber sections 2 and 5and shallower in fiber sections 3 and 4. The bottom portion of thefigure (not hatched) is a position reference for a prototype test setupwhere a pipe section, as in FIGS. 15A through 15E, is tested against thefiber buried at different depths. In the bottom portion of the figurethe pipe section is followed by control fiber laid parallel to theshallower fiber and the deeper control fiber. In section 2 the pipe testand the control fiber are laid at the same depth. In section 3 the pipetest and the control fiber are at different depths, where the controlfiber is buried closer to the surface than the pipe test. Section 4 isthe fiber control section that corresponds to the Section 3 testsection. Section 5 is the fiber control section that corresponds to theSection 2 section.

Placing coupled fibers parallel with different depth relationships mayprovide information regarding direction of sound movement, which may beused for multiple purposes such as predicting future movement.Additionally, further tests will yield how depth affects reportingsensitivity, eventually yielding data for the optimum fiber depth fordiffering mediums and fiber types.

Referring to FIG. 18, laying fiber in a triangular wave pattern providesinformation about the direction a sound is emanating from as well as thedirection and speed of travel if the sound is moving. FIG. 18 depicts anoption wherein the wave crosses over a straight fiber. The bendingradius of the fiber will be dependent on the characteristics of thefiber used as well as the size of the overall implementation. Fibercrossing fiber should provide further data regarding where the sound isemanating from as well as the direction and speed of travel. Theinformation from the crossing fiber section can be combined and/orcompared with the information retrieved from the wave section in orderto yield a more accurate estimate. While triangle wave is specificallydiscussed, other wave patterns are possible. For instance a sine wavemay be used. However, a triangle wave is simpler to implement and thecode is simpler, thus being generally less expensive than othercontemplated wave patterns. A triangle wave is proposed in place of atrue sine wave because it is simpler and less expensive to lay the fiberand generate code for a sine wave. Further, placing a coupled reflectorbeneath a section of fiber will reflect a signal back up as an echo,which can be used to triangulate the source of the sound impinging uponthe fiber and elevating detection threshold. There are many options formaterials and shapes of reflectors. A smooth material such as metal willreflect the sound more effectively than a porous material. A parabolashape will be the most effective shape as any wave impinging on it willalways reflect to the focal point of the parabolic reflector and thuselevate the detection threshold.

Higher soil hydration enhances the acoustic sensitivity of the buriedfiber therefore seeding the soil coupled with the fiber optic sensorarray with a moisture retaining substance will cause an elevateddetection threshold. One notable coupling substance that increasesacoustic sensitivity through a medium is Sodium Polyacrylate. SodiumPolyacrylate is easily obtainable, absorbs 200-300 times its mass inwater, immobile in landfills (>90% retention), biodegradable over time,non-hazardous, and the hydration reaction is reversible.

An alternative coupling medium to the Sodium Polyacrylate is standardpotting soil. Potting soil usually contains Perlite and other waterabsorbing chemicals that keep the soil moist for longer periods betweenwatering. Potting soil is readily available and relatively inexpensive.Another option is to mix some Sodium Polyacrylate into potting soil formore absorptive yet still reasonably inexpensive soil seeding.

Another option is to place the seeded soil mixture into a porous tubethat surrounds the optic fiber. The tube serves to hold the materialonto the fiber as well as prevent other materials and rocks getting tooclose to the fiber while elevating the soil hydration in the areacoupled to the pipe.

Generally, to prove these aspects of the disclosure, a systemimplemented to test the underlying theories behind various improvedmeasurement concepts, the following controlled tests were conducted perTables 1 and 2 below:

TABLE 1 Surrounding Test # Material Reflector A Standard Soil No BStandard Soil Yes C Seeded Soil No D Seeded Soil Yes E Water No

TABLE 2 Surrounding Test # Pipe Type Pipe Filler Material 1 PVC WaterStandard Soil 2 PVC Glycerin Standard Soil

In each test the microphone was placed ten inches below the top of thefiller material and centered within the test box. The speakers weresuspended above the test box. For all tests, sound was generated by thesame tone generator, at the same set of frequencies with a sine wavesampled at a rate of 44.1 kHz through the same speaker. Each tone fromthe tone generator was generated for five seconds five times for eachtest setup. Tones were generated at 50 Hz, 500 Hz, 2.5 kHz, 10 kHz, and15 kHz. The tests resulted in the fresh water (Control E) and the seededsoil with reflector (Control D) coupling tests performing exceptionallywell. The remaining tests were only small detection thresholdimprovements upon standard fiber with standard glacial loam soil. ThePVC filled with coupling material tests performed about the same forglycerin and water as couplants, both of which were only smallimprovements upon the standard fiber tests.

The results are depicted in frequency spectrograms in FIGS. 19 through25. FIG. 19 is a frequency spectrogram for prototype Test A. FIG. 20 isa frequency spectrogram for prototype Test B. FIG. 21 is a frequencyspectrogram for prototype Test C. FIG. 22 is a frequency spectrogram forprototype Test D. FIG. 23 is a frequency spectrogram for prototype TestE. FIG. 24 is a frequency spectrogram for prototype Tests A-E. FIG. 25is a frequency spectrogram for prototype Tests 1-2.

With a focus now on an example that discloses reducing noise in a lasercoupler, it is well known in the art that fiber couplers are commonlyused basic components of many fiber-optic setups. Note that the termfiber coupler is used with two different meanings. It can be an opticalfiber device with one or more input fibers and one or more outputfibers. Light from an input fiber can appear at one or more outputs,with the power distribution potentially depending on the wavelength andpolarization. It can also be a device for coupling light from free spaceinto a fiber. The term “fiber coupler” or “coupler” used herein shallrefer to couplers of the first definition and not the second. Fibercouplers are usually directional couplers, which means that essentiallyno optical power sent into an input port can go backwards into the sameport or other input ports. There is often a specification of returnloss, which indicates how much weaker the back-reflected light is,compared with the input.

If all fibers involved are single mode there are certain physicalrestrictions on the performance of the coupler. In particular, it is notpossible to combine two or more inputs of the same optical frequencyinto a single polarization output without significant excess losses,except if the optical phases of the input beams are precisely adjustedand stabilized. That means that the two inputs to be combined would haveto be mutually coherent. Multimode fiber combiners allow the powers oftwo mutually incoherent beams to be combined without a power loss.However, this will cause some loss of brightness.

In a further discussion of noise reduction, it is understood in the artthe ratio of optical power at two different wavelengths can becontinuously measured for any change and used as an indicator of fibertapping or naturally occurring event requiring further investigation.The wavelength dependent loss of a single-mode fiber such as a CorningSMF28 fiber wrapping around a metal mandrel is used as an example.Mandrel radius and wrap angle (where one turn is 360 degrees) define theamount of loss. A change in radius will produce a similar monotonicallyincreasing loss with wavelength curve but with slightly differentcurvature. A multimode fiber or other types of suitable fiber would haveother wavelength dependent loss values.

When working with very small back reflected signals, it is criticallyimportant the system mitigate as much noise sources as possible to beable to detect and demodulate back reflected signals; referring now toFIGS. 26-27. In practice, beam splitters 1600 cause a fairly significantamount of signal loss. Two-way splitters often have built in mirrors(1610 a and 1610 b) which send a portion of the signal out in a thirdundesired direction 1620, causing additional backscatter in the signal.To reduce noise, a three-way beam splitter may be used in place of thetwo-way beam splitter. Another method includes using a mandrel toattenuate unwanted signals by running fiber from the additional outputbeam and wrapping it in a tight mandrel 1650 (bent beyond the bendradius) the signal may be attenuated until it is completely absorbed.Since all of the energy being sent into the third port is absorbed, noneis reflected thus removing noise at the beam splitter 1600.

In an example for an open-ended fiber optic detection system, a falsepositive peak in the backscatter will always be present at the remoteend of the fiber due to laser power reflecting back. The higher thevalue of the laser power reflected back, the more information is lost inthe noise. Should a true detection event occur at the remote end of thespan, it would be difficult or impossible to differentiate from noise inthe backscatter. For instance, the returning light may return with anincorrect code (time delay) impinged onto it, thus providing a falsepositive at one point in the fiber, and potentially hiding the locationof a true event. It is imperative to provide a system wherein thisend-of-span backscatter may be attenuated such that true events may bedetermined at the remote end of the span, less information is lost, andreturning light is not impinged with false time delay data. To do this,another noise canceling method is disclosed for a non-reflective remoteend; a tightly wound mandrel may be used to attenuate laser power sothat it does not reflect from the remote end of the span. Addedadvantages of using a mandrel include: uniform, predictable remote endof span response; and minimized instances of false or useless data.

FIGS. 28A through 28D depict an example for an encased mandrel assembly.FIG. 28A depicts the casing 2905 which is used to protect the mandrelassembly from the environment as well as to provide additional sound andvibration dampening. The fiber 2015 enters the mandrel assembly at oneend. The fiber 2015 is wound around a cylinder within the casing 2905 toform the mandrel. The location where the fiber 2015 enters the casing2905 is protected with and interfacing mechanism 2920 which may take theform of a strain relief coil spring (as depicted).

FIG. 28B depicts the internal components of the mandrel assembly of FIG.28A. The fiber 2015 enters a first end of the casing 2905. Just insidethe casing 2905, the fiber is stripped of the cladding (bare fiber2930). The bare fiber 2930 is wrapped around a cylinder 2910 evenly andwith no overlaps. The crushed end of the bare fiber 2930 is insertedinto a hole 2935 in the cylinder 2910. The characteristics of theparticular fiber used will dictate the length of fiber to be wound onthe mandrel, the diameter of the cylinder, and/or the number of wraps.After the fiber has been wrapped and secured, one or more screws 2915are tightened at the second end of the casing 2905 to fully lock theinternal components into place.

FIG. 28C depicts a view of the cylinder 2910 around which the bare fiber2930 is wrapped to form the mandrel of FIG. 28A. FIG. 28D depicts anexample for a securing mechanism for locking the cylinder within themandrel assembly of FIG. 28A after the fiber has been wound. Thecylinder 2910 is secured with one more screws 2915. Other securingmechanisms are contemplated. The fiber is terminated in 2945.

FIG. 29 is an overall system diagram depicting a method for generatingsuperimposed waves. A light source 405 generates a first signal 3000.The first signal 3000 travels into a splitter 410 where it is split intoa reference signal 3010 and an interrogation signal 3020. The referencesignal 3010 is modulated by a first optical modulator 415. Theinterrogation signal 3020 is modulated by a second optical modulator420. The modulated interrogation signal 3030 enters a first connection3005 of a circulator 430. The modulated interrogation signal 3030 istransmitted from a second connection 3015 on the circulator 430 into afiber under test 505. The circulator 430 receives at the secondconnection 3015 a modulated signal backscattered 3040 from the fiberunder test 505. The circulator 430 transmits the modulated signalbackscattered 3040 from the fiber under test 505 from a third connection3025 into a mixer 525. The mixer 525 receives the modulated referencesignal 3050 from the first optical modulator 415. The mixer 525 mixesthe modulated reference signal 3050 and the modulated signalbackscattered 3040 from the fiber under test 505 into superimposed waves3060.

For the sake of convenience, the operations are described as variousinterconnected functional blocks or distinct software modules. This isnot necessary, however, and there may be cases where these functionalblocks or modules are equivalently aggregated into a single logicdevice, program or operation with unclear boundaries. In any event, thefunctional blocks and software modules or described features can beimplemented by themselves, or in combination with other operations ineither hardware or software.

Having described and illustrated the principles of the inventionthereof, it should be apparent that the invention may be modified inarrangement and detail without departing from such principles. Claim ismade to all modifications and variation coming within the spirit andscope of the following claims.

What is claimed is:
 1. An integrated fiber optic interferometryinterrogator system to generate superimposed waves, the systemcomprising: an optical light source, wherein the optical light sourcegenerates a first signal; a first signal splitter coupled to an outputof the optical light source, wherein the first signal splitter splitsthe first signal into a reference signal and an interrogation signal; afirst optical modulator to modulate the reference signal; a secondoptical modulator to modulate the interrogation signal; a fiber couplerconnected to a fiber under test; a circulator comprising a plurality ofconnections, wherein a first connection of the plurality of connectionsreceives the modulated interrogation signal, a second connection of theplurality of connections transmits the modulated interrogation signalthrough the optical coupler and receives from the optical coupler amodulated signal backscattered from the fiber under test, and a thirdconnection of the plurality of connections that transmits through anisolator the modulated signal backscattered from the fiber under test; asignal mixer comprising a plurality of signal couplers configured toreceive the modulated signal backscattered from the isolator connectedto the fiber under test, receive the reference signal modulated by thefirst optical modulator, mix the two received signals into thesuperimposed waves, and transmit the superimposed waves into a pluralityof photo diodes, wherein one or more of the photo diodes output an RFsignal comprising the superimposed waves.
 2. The system of claim 1,wherein the light source is a laser.
 3. The system of claim 2, whereinthe laser is one of continuous wave (CW) and pulse modulated CW.
 4. Thesystem of claim 1, wherein the first optical modulator is at least oneof acousto-optical modulator and electro-optical modulator and whereinthe second optical modulator is at least one of acousto-opticalmodulator and electro-optical modulator.
 5. The system of claim 1,wherein the fiber under test comprises at least one of a single-modetype, multimode type, and polarization preserving type fiber opticcable.
 6. The system of claim 1, wherein the signal couplers are atleast one of signal splitter and signal combiner.
 7. The system of claim1, wherein the fiber under test has a length L and the light source is alaser having the capability to generate a signal with sufficientstability to retain coherency in propagation along the fiber under testfor a distance at least equal to two times the length L.
 8. The systemof claim 1, wherein the fiber under test has a coating thereon made of athermoplastic material having the combined characteristics of a lowYoung's modulus and a Poisson's ratio below that of natural rubber,wherein the coating enhances the longitudinal component of strainvariation derived from an acoustic wave signal.
 9. The system of claim1, wherein the plurality of photo diodes are connected to a plurality ofamplifiers and wherein the amplifiers generate radio frequency signals.10. The system of claim 1, further comprising a power supply.
 11. Thesystem of claim 1, wherein the system elements are at least one ofoptimally coupled, connected, and linked for at least one of maximumefficiency and lowest loss, wherein maximum efficiency includes at leastone of cable length and number of splices.
 12. The system of claim 1,wherein all signal splitters and signal couplers use signal attenuatorson all unused ports.
 13. The system of claim 12, wherein the signalattenuator is a mandrel.
 14. The system of claim 1, wherein the systemis enclosed by a material conducive for vibration attenuation.
 15. Thesystem of claim 1, wherein the modulated backscattered signals aregenerated from at least one of acoustic pressure waves, electromagneticfields, mechanical strain or pressure, and thermal strain or pressure.16. A method for generating superimposed waves with an integrated fiberoptic interferometry interrogator system, the method comprising:generating a first signal; splitting the first signal into a referencesignal and an interrogation signal; modulating the reference signalusing a first optical modulator; modulating the interrogation signalusing a second optical modulator; using a circulator comprising aplurality of connections operative to receive at a first connection themodulated interrogation signal, transmit from a second connection themodulated interrogation signal into a fiber under test, receive amodulated signal backscattered from the fiber under test, and transmitfrom a third connection the modulated signal backscattered from thefiber under test; using a signal mixer comprising a plurality of signalcouplers to: receive the modulated signal backscattered from the fiberunder test, receive the modulated reference signal from the firstoptical modulator, mix the two received signals into the superimposedwaves.
 17. The method of claim 16, wherein the fiber under test has alength L and the light source is a laser having the capability togenerate a signal with sufficient stability to retain coherency inpropagation along the fiber under test for a distance at least equal totwo times the length L.
 18. The method of claim 16, wherein the fiberunder test has a coating thereon made of a thermoplastic material havingthe combined characteristics of a low Young's modulus and a Poisson'sratio below that of natural rubber, wherein the coating enhances thelongitudinal component of strain variation derived from an acoustic wavesignal.
 19. The method of claim 16, wherein the system is enclosed by amaterial conducive for vibration attenuation.
 20. The method of claim16, wherein the modulated backscattered signals are generated from atleast one of acoustic pressure waves, electromagnetic fields, mechanicalstrain or pressure, and thermal strain or pressure.